Recently, an electrical resistivity survey has been widely used as one of geophysical surveys for non-destructively imaging a subsurface. The electrical resistivity survey is a geophysical method that artificially applies an electric current I to the subsurface so as to measure electric potential ΔV and then surveys a geological structure or an anomaly zone on the basis of difference in distribution of electrical and physical properties between subsurface mediums, thereby imaging a stratum structure formed with a non-uniform and complicated medium.
In other words, while a direct current flows alternately between forward and backward through a pair of electrodes, the electric potential is measured in another pair of potential electrodes, thereby surveying distribution of subsurface electrical resistivity.
FIG. 1 is a view for explaining field measurement using an electrical resistivity surveying method capable of non-destructively imaging a subsurface on the basis of difference in subsurface electrical conductivity in the case that the subsurface electrical conductivity is differently distributed according to geological features.
Specific resistance is reciprocal quantity of electrical conductance that is a measure of how easily electricity flows, and has a unit of ohm-meter, symbol Ω-m. As a result of such an electrical survey, if a part corresponding to low specific resistance appears in a cross-section of the imaged structure, it is generally determined as a mineral vein or a water vein. Further, in case of a site investigation or the like performed before tunnel excavation under recent civil engineering construction, such a part corresponding to the low specific resistance refers to a weak zone such as a fracture zone or a fault, etc., which has to be avoided or of which a counterplan has to be considered previously.
Besides, the electrical resistivity survey has recently been applied to not only a survey of an environment pollution zone in a landfill or an oil-leak place, but also an archaeological investigation.
An electrical resistivity survey system includes a transmitter, a receiver, a digital controller, a power supply, measuring software, etc. Additionally, for automatic measurement, the electrical resistivity survey includes a multi-channel relay based on digital output, a control circuit for a distributing board, and wiring.
FIG. 2 illustrates an example of a measuring system for the electrical resistivity survey.
In the receiver, receiving electrodes P1, P2 measure potential difference changed while an electric current flows from the transmitter between current electrodes C1, C2 through an underground medium. Then, an analog-to-digital (A/D) converter converts the potential difference to a digital signal as its gain is controlled. The digital signal is transmitted to a control/measurement notebook personal computer (PC) through a serial communication line.
At first thought, it may be regarded that such an electrical survey apparatus, which measures voltage by flowing the electric current in the subsurface while moving on the Earth's surface and finds a part where electricity easily flows, is one of electronic devices consisting of common electronic components. However, without design based on right theory and practical experience, it is not easy to make the surveying apparatus that will be satisfactorily usable in various fields.
The electrical resistivity survey has an electrode configuration such as a pole-pole array, a dipole-dipole array, a Wenner array, a Schlumberger array, etc. Every electrode configuration has advantages and disadvantages. Therefore, the best way is to compare the surveys performed with various electrode configurations on the same measuring line and complement the advantages and disadvantages of the electrode configurations.
However, it is difficult to practically apply various methods in consideration of time, economical efficiency or field conditions. Thus, there is a need of selecting an optimum electrode configuration according to objects, scales and field conditions of the survey.
As one of the geophysical surveys, a testing method and a principle of the electrical resistivity survey are as follows: the potential difference due to resistivity anomaly existing in the subsurface medium is measured and analyzed, and thus a subsurface structure such as a geological structure, a fault or a fracture zone, groundwater, etc. is examined.
In the measuring system for the electrical resistivity survey, the electrodes are selected one pair by one pair among several tens of electrodes and then the transmission and the measurement are performed. Recently, such a series of manual measurement has been automated by a relay configuration under software control.
In this case, use of the relay is limited to hundreds of thousands of electric contacts, so that the durability of the relay is not permanent. Further, the relay may easily break due to malfunction. Also, the relay becomes larger as the transmitting current capacity increases.
Particularly, if a relay controller is suddenly turned off while applying high voltage to an output-side load, mechanical contact points of the relay are largely damaged. This should be considered in designing the electrical survey because it often happens to the electrical survey that is an outdoor survey employing a rechargeable battery. To make provision against the foregoing damage, the relay may have a sufficiently large capacity. However, this case not only increases the size and weight of the relay, but also increases a unit cost and a driving power thereof. Further, the relay in this case still has a mechanical limit to electric contact, so that the problem of impermanent durability cannot be solved. In other words, the relay is not permanent because the use of the relay is limited to hundreds of thousands of electric contacts. Actually, the relay for the electrical survey operates to have one electric contact per second, so that the operation of the relay exceeds a million electric contacts for just several months. Thus, the relay is often used up.
Besides, the relay has problems of low speed, a contact noise owing to arc generation, a surge noise due to inductive load, mechanical wear, bounce chattering, etc.
Accordingly, if there is something that employs a configuration of a semiconductor (MOSFET or the like) to replace the relay for automatically controlling the electrode configuration, it will be used as a very stable and semipermanent apparatus. However, at present, there is no existing apparatus which applies the configuration of the semiconductor.